1. Field of the Invention
The present invention relates to a compression rate control method in a digital recording apparatus such as a digital VTR.
2. Background of the Invention
In a digital VTR, input image data is recorded on a video tape after it is compressed by an image compressing circuit such as a JPEG (Joint Photographic Experts Group) compressing circuit. When a compression rate in the image compressing circuit is increased, an amount of coding after the compression is decreased. When the amount of coding is small, a recording time period during which the data can be recorded on the video tape is lengthened. On the other hand, when the compression rate is decreased, the amount of coding after the compression is increased. When the amount of coding is large, the recording time period during which the data can be recorded on the video tape is shortened.
Used as the digital VTR is one capable of selecting a recording mode corresponding to the recording time period, for example, a normal time mode, a longtime mode, and a 12-hour recording mode. In the digital VTR, the amount of coding after the compression must be converged on a predetermined value depending on each of the recording modes.
For example, when image data corresponding to 12 hours is recorded on the video tape, an amount of coding per field for satisfying the recording mode is calculated from a compressing method and a recording format. The compression rate is calculated using the calculated amount of coding as a target amount of coding per field.
Suppose a case where an image obtained by subjecting images respectively picked up by a plurality of monitoring cameras, four monitoring cameras A, B, C, and D, to time division multiplexing is recorded by one digital VTR. Since each of the monitoring cameras is normally fixed, the images respectively picked up by the monitoring cameras differ, while the image picked up by one of the monitoring cameras is not so changed in many cases.
Letting a, b, c, and d be amounts of coding per field in cases where the images picked up by the monitoring cameras A, B, C, and D are compressed at the same compression rate. It is assumed that the amount of coding a is the smallest of the amounts of coding a, b, c, and d.
In such a case, when a compression rate corresponding to a target amount of coding dependent on the recording mode is calculated using the amount of coding a, which is the smallest, of the monitoring camera A, for example, as a basis, and the compression rate is also applied to a case where the images picked up by the other monitoring cameras B, C, and D are compressed, it is a compression rate suitable for an input signal from the monitoring camera A, while not being a compression rate suitable for input signals from the other monitoring cameras A, B, C, and D.
It is also considered that the average of the amounts of coding a, b, c, and d per field by the monitoring cameras A, B, C, and D is found, to calculate a compression rate corresponding to the target amount of coding dependent on the recording mode using the obtained average as a basis. However, it is not a compression rate suitable for each of the input signals from the monitoring cameras B, C, and D.